1. Technical Field
This disclosure is directed to compressor control, and more particularly, to control algorithms for improving the performance of centrifugal compressors under part load.
2. Description of the Related Art
Centrifugal compressors are well known in the art of refrigeration systems and are typically designed to rotate at fixed or predetermined operating speeds. Capacity control of the machine is normally accomplished by varying the position of a series of adjustable guide vanes located at the inlet of the machine. The mass rate of flow of refrigerant delivered to the impeller is thus varied to meet the changing load demands made on the machine. At maximum flow, the refrigerant leaving the impeller may be more than the diffuser can handle and the flow may become choked at the diffuser. At lower flow rates, the flow of refrigerant moving through the diffuser may become unstable and a partial flow reversal may take place, resulting in noise and a dramatic reduction in machine efficiency. Eventually a complete reversal in flow is experienced whereupon the compressor may stall or surge. The range between a choke condition and the onset of a surge condition generally defines the operating range of a compressor. In a compressor relying solely upon the inlet guide vanes for capacity control, this range is narrow, particularly when vanes are used in the diffuser.
Accordingly, one of the major problems associated with the use of centrifugal compressors is that of maintaining flow stabilization when the compressor load varies over a wide range. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate. When there is a relatively low volumetric flow rate through such a compressor, the flow becomes unstable. More specifically, as the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there occurs a partial reversal of flow in the diffuser passage, creating noise and lowering the compressor efficiency. Below this range, the compressor encounters surge, wherein complete flow reversals in the diffuser passage take place, destroying the efficiency of the machine and endangering the integrity of the machine elements.
Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates. The prior art has attempted to control surge in centrifugal compressors by providing variable guide vanes that control the flow of refrigerant through the compressor. While this technique has helped, it has not been completely effective, as fluid flow through the diffuser accounts for most of the conditions that cause surge. Similarly, variable geometry pipe or vaned diffusers for centrifugal compressors have been disclosed to provide improved control of compressor performance under changing loads, and further, to avoid instances of surge. For example, a variable diffuser may include inner and outer rings having complementary inlet flow channel sections formed therein. The inner and outer rings may be rotatable with respect to one another, thereby allowing for the adjustment of fluid flow through the flow channels.
Among comparable compressors, centrifugal compressors with vaned or piped diffusers have the highest efficiency at full load. However, the surge margin is relatively low at part load. Variable diffusers may be used to overcome drawbacks associated with the low surge margin at part load. Moreover, variable diffusers are generally adjusted according to a fixed relationship between the inlet guide vane and the diffuser to achieve the highest surge margin. However, the resulting efficiency of a compressor optimized for surge at part load is poor when operated along a desired load line for a particular geographic region. Performance may be improved by using a schedule specifically dedicated to a given load line. However, improved performance at part load comes only at the expense of a lower surge margin.
Accordingly, there is a need for an improved method for controlling a centrifugal compressor that maintains full load performance and efficiency, and significantly improves part load performance and efficiency without adversely affecting the surge margin.